[vsnet-grb-info 20533] GRB 180410A: Swift-XRT refined Analysis

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22629
SUBJECT: GRB 180410A: Swift-XRT refined Analysis
DATE:    18/04/10 23:40:51 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P.A. Evans (U. Leicester), S.L. Gibson (U. Leicester), Z. Liu (NAOC /
U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri
(INAF-OAB), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N.
Burrows (PSU) and J.D. Gropp report on behalf of the Swift-XRT team:

We have analysed 6.8 ks of XRT data for GRB 180410A (Gropp et al. GCN
Circ. 22620), from 176 s to 45.6 ks after the  BAT trigger. The data
comprise 201 s in Windowed Timing (WT) mode with the remainder in
Photon Counting (PC) mode. The enhanced XRT position for this burst was
given by Evans et al. (GCN Circ. 22622).

The light curve can be modelled with a series of power-law decays. The
initial decay index is alpha=1.39 (+0.22, -0.28). At T+321 s  the decay
steepens to an alpha of 2.9 (+0.4, -0.3) before breaking again at T+866
s to a final decay with index alpha=1.48 (+0.15, -0.13).

A spectrum formed from the WT mode data can be fitted with an absorbed
power-law with a photon spectral index	of 1.87 (+0.07, -0.05). The
best-fitting absorption column is  consistent with the Galactic value
of 6.6 x 10^21 cm^-2 (Willingale et al. 2013). The PC mode spectrum has
a photon index of 1.45 (+0.21, -0.19) and a best-fitting absorption
column of 7.7 (+2.3, -1.2) x 10^21 cm^-2. The counts to observed
(unabsorbed) 0.3-10 keV flux conversion factor deduced from this
spectrum  is 6.2 x 10^-11 (8.1 x 10^-11) erg cm^-2 count^-1. 

A summary of the PC-mode spectrum is thus:
Total column:	     7.7 (+2.3, -1.2) x 10^21 cm^-2
Galactic foreground: 6.6 x 10^21 cm^-2
Excess significance: <1.6 sigma
Photon index:	     1.45 (+0.21, -0.19)

If the light curve continues to decay with a power-law decay index of
1.48, the count rate at T+24 hours will be 6.1 x 10^-4 count s^-1,
corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.7 x
10^-14 (4.9 x 10^-14) erg cm^-2 s^-1.

The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00824063.

This circular is an official product of the Swift-XRT team.